The invention relates to start switch control circuitry for automatically connecting and disconnecting a start capacitor of a single phase AC induction motor to and from the AC power source in starting and running modes, respectively. The invention prevents the start capacitor from being energized when the motor is rotating or coasting above a given speed, as may occur upon a momentary power line loss.
The invention particularly relates to improvements in reduced cost and part content over commonly owned co-pending U.S. application Ser. No. 911,012, filed herewith, "Start From Coast Protective Circuit", Chmiel, hereby incorporated herein by reference. The invention is particularly useful as an optional addition to the simplified circuitry shown in commonly owned co-pending U.S. application Ser. No. 06/885,087, filed July 14, 1986, "Low Cost, Load And Speed Sensitive Motor Control Starting Circuit", Wrege et al, now U.S. Pat. No. 4,670,697, hereby incorporated herein by reference, which provides improvements in reduced cost and part content over commonly owned co-pending allowed U.S. application Ser. No. 680,489, filed Dec. 11, 1984, "Load And Speed Sensitive Motor Control Starting Circuit", Shemanske et al, now U.S. Pat. No. 4,662,506, hereby incorporated herein by reference. The invention is particularly desirable in motors above a given horsepower rating, e.g., two horsepower and above.
If the motor is de-energized while in the running mode, either by a power loss or by intentionally switching the power off, the motor will continue to rotate and coast, particularly if it is subject to a high inertial load. If the start capacitor were to be energized under these conditions, a large fault current might flow through the auxiliary winding, the start capacitor and the start switch circuitry, potentially damaging the start switch circuitry and the start capacitor. While the motor is coasting, motor rotation induces a voltage in the auxiliary winding. This induced voltage plus any applied voltage from the power line causes the potentially damaging current. Burn-out of the start capacitor and/or start switch is a particular problem experienced during momentary power line loss where electrical power is reapplied from the line to the start capacitor while the motor is still rotating above a given safe cut-out speed.